The hexagonal polar ZnO(0001)-(1×1) surfaces: structural features as stemming from X-ray diffraction

The ZnO surfaces, apart from their role in catalytic processes, raise fundamental questions regarding the key parameters, which stabilise ionic material surfaces. The ZnO structure consists of hexagonal planes stacked with the sequence aBbAaB… The polar (0001) surfaces should be unstable because of the normal dipole moment. However, a long-range (1×1) order is obtained after several Ar+-800°C cycles. We investigated, by grazing incidence X-ray diffraction (XRD), both the O- and Zn-terminated faces, issued from a single substrate. In both cases, the coherent domain width was found between 180 and 100 A. This is the average size of the terraces, limited by bilayer steps, and thus either a or b terminated. Several intensity rods (along the surface normal) were analysed, attesting that the surface is close to bulk truncation (no stacking fault). The effects of relaxation or non-stoichiometry are strongly damped by the contribution of the two types of terraces. In the Zn case, the atomic displacements are not significant (outward relaxation of +0.05 A), whereas in the O case, four layers have to be displaced (inward relaxation by −0.3 A). The fits are greatly improved allowing for partial occupancy in the topmost planes: 0.75 for the external plane in the Zn case, a result which fits well with the electrostatic arguments for the cancellation of the dipole moment, and 1.3, 0.7, for the two topmost planes on the O surface. Impurity or Zn atoms substituting the O atoms could explain these latter values.